1. Field of the Invention
The present invention relates generally to piezoelectric positioning devices and in particular to piezoelectric positioning devices with reversible action of both rotational and linear reciprocal types. More specifically, the positioning device of the invention contains one or more actuators having a linear contact with the corresponding rotor or the slider. The positioning device of the invention can be used generally as a high performance replacement for small conventional electric drivers used in computer equipment, robotics, manufacturing equipment, aerospace, automobiles, toys, etc. The positioning device of the present invention can be used particularly effectively in various turntable and disk drive devices such as for magnetic and optical data storage equipment, especially when small size and weight are required for compactness and when precision positioning of a magnetic or optical reading head is needed for accurate and reliable performance.
2. Description of the Prior Art
Linear and rotational piezoelectric positioning devices with reversible action are generally well known and widely used in various applications. All of these known devices incorporate a basic piezoelectric drive arrangement in which a piezoelectric actuator is placed in contact with a movable element such as a slider or a rotor made typically of a hard to compress material. A frictional contact surface is therefore intermittently present between the actuator and that movable element which transmits the driving force or torque from the piezoelectric actuator to the movable element. Limitations of such arrangement are described in detail in the parent patent applications. Briefly, these devices have limited number of operating hours due to the wear and ultimately mechanical failure of the surface of the movable element. Once the surface is damaged, the contact between the actuator and the movable element is no longer optimal and the driving force transmission is no longer present. One negative consequence of that is reduced accuracy of positioning as well as reduced operational life. Another limitation of these devices is in the relatively high clamping force between the actuator and the movable element in an attempt to partially compensate for the wear of the surface contact. That high clamping force reduces the efficiency of the force transmission and overall efficiency of the device.
We have proposed and described in details in our previous applications the piezoelectric drive device with linear contact between the actuator and the movable element. Briefly stated, the actuator equipped with a hard surface insert engages with the compressible surface of the movable element to compress it within the elastic limits thereof without any slippage and to form a temporary microgroove on that surface. Upon disengagement, the surface of the movable element restores its initial shape until the next compression cycle. These devices demonstrate higher energy transmission efficiency and longer operating life. However, reversible devices with such linear contact, both linear and rotational were not known before in the prior art.
One particularly advantageous application of a reversible device of this type is for data storage devices. Those skilled in the art of making and using data processing and storage equipment are familiar with magnetic data storage arrangements when a transducer is positioned adjacent a moving magnetic recording surface. Such a device will record magnetic bits as data from the disk surface and recover this information by processing the signal from a transducer adjacent a particular xe2x80x9crecording trackxe2x80x9d along the surface.
This invention relates in part to a piezoelectric positioning device for establishing and maintaining the placement of such a transducer with respect to such recording tracks; it is particularly adapted for recording on magnetic tape, drum, and disk media, especially for high density, high TPI recording as well as for recording on various optical disks and other optical devices. In such recording, a fast, non-magnetic, miniaturized, solid state translation means is particularly desiredxe2x80x94especially where translation distances are relatively small, on the order of a few dozen micro inches or more, typically over a total excursion of only a few mils.
Limitations in present-day transducer positioning apparatus, such as the typical voice coil actuator systems, or the like are well known. In addition to their mechanical inefficiency due to a large number of moving parts, such systems are undesirably large, slow and inflexible in their design approach. They are particularly unsatisfactory for xe2x80x9ccenteringxe2x80x9d a transducer relative to a narrow recording track, where positioning is critical. Such systems are also troublesome in that they use solenoid magnets or other magnetic actuator means, creating stray magnetic fields that can interfere with the magnetic recording apparatus. The present invention is adapted to improve these shortcomings with a solid state, piezo-electric flexure arrangement for mounting and positioning magnetic heads.
Various reversible piezoelectric positioning devices were proposed in the prior art for use in this disk drive application. Examples of such devices can be found in U.S. Pat. Nos. 4,188,645 by Ragle; 4,764,828 by Gollbach; 5,189,578 by Mori; 5,400,192 by Mizoshita; 5,438,469 by Rudi; 5,521,778 by Boutaghou and others. As was indicated above, all of these devices suffer from the limitations arising from the surface contact between the piezoelectric actuator and the movable element, a rotor or a swinging arm in this case. A smaller and more efficient positioning device with extended operational life is therefore needed for these disk drives.
Various reversible piezoelectric devices were proposed by Zumeris of Nanomotion and described in the following U.S. Patents which are incorporated herein by reference: Nos. 5,616,980; 5,682,076; 5,696,421; 5,714,833; and 5,777,423. The design of a piezoelectric plate is of particular interest for this invention. Generally speaking, the piezoelectric plate is described as having a rectangular shape. Four electrodes are plated or otherwise attached to the top face of the plate in a checkerboard alternating arrangement. A larger single electrode is placed on the opposite bottom face of the plate. By exciting the top and the bottom electrodes in alternate sequence, one can achieve certain vibrations of the plate in either one of the opposite directions. These vibrations can then be used to reversibly drive a movable element. Although the design of the piezoelectric plate has certain advantages such as simplicity of operation, these devices have similar limitations to those of the other patents of the prior art. The most important limitation is limited operational life and low efficiency of energy transmission due to the wear and eventual slippage of the contact surface of the movable element made of a hard to compress material. That leads to reduced accuracy and lower efficiency of operation. Piezoelectric reversible positioning device is therefore needed to overcome these limitations.
Accordingly, it is an object of the present invention to overcome these and other drawbacks of the prior art by providing a novel reversible piezoelectric positioning device of both rotational and linear type with linear contact between the actuator and the movable element with improved operational life.
Another object of the invention is to provide a reversible piezoelectric positioning device for moving the arm containing a reading head of a disk drive for data storage devices such as optical, CD-ROM, hard drive, floppy drive, and other similar magnetic and optical disks and tapes; such positioning device being more accurate in positioning, smaller in size, lower in weight, and requiring less energy for its operation.
Yet another object of the present invention is to provide a reversible piezoelectric positioning device with superior start-stop characteristics and reduced response time, preferably such response time being less than 2 msec.
A further object of the invention is to provide a reversible piezoelectric positioning device with improved positioning accuracy and resolution.
A further yet object of the invention is to provide a reversible piezoelectric positioning device with one or more actuators.
A further yet object of the invention is to provide a reversible piezoelectric positioning device with an actuator adapted to compresses the surface of the movable element within elastic limits thereof, in which case the surface of the movable element fully restores its initial shape after the compression from the actuator is removed.
A final object of the present invention is to provide a control method for a reversible piezoelectric positioning device with improved positioning resolution and accuracy by supplying alternate electrodes of the piezoelectric plate with electrical signal of different voltage.
According to the invention, the reversible positioning device contains generally a piezoelectric plate equipped with a set of electrodes. The plate is capable of generating vibrations once voltage is supplied from the control unit through respective electrodes. These vibrations generate the plate flexing motion which can be directed in either one of two opposite directions depending on which electrodes are activated. The plate is equipped also with a hard edge insert of a linear type which is urged against a movable element. The plate with the hard edge insert is preferably positioned perpendicularly against the movable element. Once appropriate electrical voltage is supplied to either one of the alternate electrodes, the flexing motion of the piezoelectric plate is transferred through the hard edge insert onto the movable element through elastic compression thereof. The movable element then moves a predetermined distance of a single step in a predetermined direction after which the hard edge insert is disengaged from the compressible surface of the movable element and the device is ready for the next movement cycle. Reverse direction of movement can be achieved by switching the polarity of the electrical signal and activating the other set of electrodes.
More specifically, the interaction between the piezoelectric actuator and the movable element is based on a contact between a hard edge insert of the actuator having a Young""s modulus Ea in the range of between 1xc3x97107 N/cm2 and 10xc3x97107 N/cm2 and preferably about 5xc3x97107 N/cm2 and a generally softer and more elastic surface of the movable element having a Young""s modulus Ee in the range of between about 0.5xc3x97106 N/cm2 and about 5xc3x97106 N/cm2 and preferably about 2xc3x97106 N/cm2. Such difference between the hardness of the hard edge insert of the actuator and the surface of the movable element leads to a particularly advantageous interaction between these two parts of the device. Engagement of the hard edge insert with the surface of the movable element leads to an elastic compression of that surface to a certain predetermined depth forming a microgroove during the time when both parts move together. Subsequent contraction of the actuator disengages both parts and the elastic surface of the movable element restores its initial shape. During the elongation/contraction cycle, the hard edge insert of the actuator undertakes a complex geometrical motion, trajectory of which is a result of both the frictional and elastic compression interaction between the actuator and the movable element. Should the phase shift between the longitudinal and the bending oscillations of the actuator be about xcfx80/2, this trajectory becomes continuous and close to the shape of an oval. In addition to the friction and elastic deformation of the movable element by the actuator, a wedging effect occurs when the contact angle is about 45 degrees which can be utilized to further increase the driving force by about additional 30 to 40%.
In accordance with the present invention, extended operational life of the device can be achieved by ensuring the depth of compressions of the movable element not exceeding its elastic limit. In that case, every compression of the movable element by the actuator is purely elastic and no permanent deformation occurs which may lead to premature wear and surface damage. Elastic materials for the movable element and the main parameters of the actuator are chosen in such a way that the relative deformation xcex5 along the line of contact of the movable element surface does not exceed about 0.001.
Another benefit of the wedging of the actuator into the relatively softer surface of the movable element is that there is less slippage between the contact surfaces of the two components than when there is only frictional contact. Slippage between the contact surfaces increases when a load is placed against the movable element and when the device is initially turned on, until it reaches its resonant frequency. Higher slippage rates are associated with declining efficiency and positioning accuracy. It is not uncommon in the art to employ a sensor on the movable element to relay positioning information to the control unit for purposes of eliminating the positioning inaccuracies caused by slippage between the contact surfaces.
Increased driving force may be also achieved by employing two or more actuators. In that case, multiple piezoelectric actuators are interacting with a single movable element preferably all at the same time allowing for increase of the device loading force without changing the dimensions and weight.
Since the hard edge insert formes a linear microgroove when engaged with the surface of the movable element, the slippage of the insert relative to the movable element is largely avoided so the accuracy of positioning is maintained throughout the operational life of the device. Also, the clamping force may be substantially reduced in this case which allows for general reduction of the sizes and weight of all involved elements without sacrificing in performance. In turn, reduced weight allows for faster response time of less than 2 msec since inertia of all movable elements is substantially reduced.
A further improvement is a novel control method for a device of this type. To improve the resolution of the reversible positioning device, one can apply different voltages to both xe2x80x9cforwardxe2x80x9d and xe2x80x9cbackxe2x80x9d steps and alternate them at the same time. The result is such that the movable element will first move in say xe2x80x9cforwardxe2x80x9d direction to the distance determined by the xe2x80x9cforwardxe2x80x9d voltage and then right after that move xe2x80x9cbackxe2x80x9d to a distance determined by a lower xe2x80x9cbackxe2x80x9d voltage. The resultant distance of travel can therefore be finely determined by a difference between the xe2x80x9cforwardxe2x80x9d and the xe2x80x9cbackxe2x80x9d distances which in turn are determined by the xe2x80x9cforwardxe2x80x9d and xe2x80x9cbackxe2x80x9d voltages. This control method of one step xe2x80x9cforwardxe2x80x9d and smaller step xe2x80x9cbackxe2x80x9d allows to achieve extremely high positioning accuracy and resolution of as low as 0.2 micron while taking all advantages of the low weight and inertia of the positioning device of the present invention.